drm/ast: Free container instead of member in ast_user_framebuffer_destroy()

Technically freeing ast_fb->base is the same as freeing ast_fb as 'base'
the first member of the data structure.
Still this makes it cleaner.

Signed-off-by: Egbert Eich <eich@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>